In various kinds of apparatuses utilizing fluids (for example lubrication oil), fine particulate impurities may find their way into the fluid. If such impurities are not removed, the apparatus, such as an engine, may be damaged. To avoid such catastrophic failures, various kinds of filtering systems have been proposed. In the most usual filtering systems, a filter is commonly inserted into the main circulation system. Main circulation system filters generally have a low density.
Heavy-duty diesel engine life, or time to rebuild, has historically been linked directly to piston ring, cylinder liner or crankshaft bearing life. Engine design parameters require that these engine components be hydrodynamically lubricated, i.e., operate with a film of lubricant separating these engine components from associated metal surfaces. Consequently, the principle mechanism associated with piston rings, cylinder liners and crankshaft journal bearings wear is not metal-to-metal contact or frictional wear. The primary diesel engine wear mode influencing engine life is corrosive wear caused by sulfur and nitrogen containing acids formed during the diesel fuel combustion process. One estimate is that more than 70 percent of heavy-duty diesel engine wear is caused by combustion acid metal corrosion.
Control of diesel engine corrosive wear has historically been accomplished through inclusion of basic or alkaline chemicals within the engine oil that is utilized to form the hydrodynamic lubricant film. These alkaline components rapidly neutralize or solubilize combustion acids upon contact with the acid molecules. The effectiveness of the corrosive wear control is entirely dependent upon the probability of the acid being neutralized by alkaline oil components prior to contact of the acid with engine metal surfaces resulting in corrosive wear. The amount of engine corrosive wear can typically be monitored through the use of oil analysis where cylinder liner wear is associated with iron parts per million (ppm) level in the engine oil. Piston ring wear is monitored by chromium levels and crankshaft bearing wear is reflected by lead levels in the oil.
The corrosive wear process begins in the diesel engine combustion chamber where the hydrocarbon diesel fuel containing sulfur compounds is combusted in the presence of oxygen and nitrogen. The hydrocarbon fuel is converted to principally carbon dioxide and water, creating extremely high gas pressures, which push down on the top of the piston to produce engine power. Also produced are SOx and NOx compounds, which rapidly react with the water released during fuel combustion yielding primarily sulfuric acid and nitric acid. These acids reach engine metal surfaces by direct contact in the cylinder bore or as blow-by gases as a normal part of engine operation. The hydrodynamic lubricant film present in the piston ring belt zone will also transport acid molecules throughout the engine as the lubricant is constantly circulated.
Combustion acid neutralization is completed using a simple acid-base reaction where metal carbonates carried as alkaline components within the lubricant directly react with sulfuric and nitric acids. The effectiveness of corrosive wear control is totally dependent upon the probability of these metal carbonates coming in contact with the acid molecules before these same molecules contact engine metal surfaces. Another factor influencing the rate and efficiency of acid neutralization is acid solubilization within the lubricant by another oil additive classified as an ashless dispersant. Dispersants are long chain hydrocarbon polymers, which are functionalized by terminating the polymer chain with a functional group generally containing basic nitrogen. Dispersants will rapidly complex with combustion acids dispersing or solubilizing them within the lubricant for transportation to a metal carbonate where the acid is converted to a neutral metallic salt. The combined efficiency of dispersant acid complexing and metallic carbonate acid neutralization controls the rate of engine wear.
Overbased or alkaline metallic detergents have been widely utilized as metallic carbonate carriers in diesel engine oil compositions. Calcium and magnesium sulfonates and phenates account for the majority of the detergents utilized to formulate diesel engine oils. Overbased detergents are produced by incorporating extra calcium or magnesium within a physical structure called a detergent micelle. For example, alkylbenzenesulfonic when reacted with calcium hydroxide and blown with carbon dioxide during the reaction process will produce an overbased calcium sulfonate. The extra metal or calcium present in the detergent micelle structure is calcium carbonate surrounded by oil solubilizing calcium sulfonate detergents. This physical structure circulating within the oil delivers the calcium carbonate to the combustion acid molecules for acid neutralization.
Ideally, there should be no limit to the amount of alkaline detergent incorporated within a diesel engine oil formulation; however, in reality modem diesel engines can only tolerate a limited level of metallic detergents before metallic ash deposits cause piston ring sticking and exhaust valve guttering. These ash deposits are caused by pyrolysis of oil metal organo compounds, principally calcium and magnesium detergents.
Recognizing (1) most diesel engine wear is caused by acid corrosion, (2) the lubricant ash content is limited, and (3) newer diesel engine designs will incorporate exhaust gas recirculation where combustion acids will be concentrated and reintroduced into the engine, a system capable of neutralizing combustion acids without significantly altering diesel engine oil compositions while at the same time filtering solid impurities from the oil circulation system would significantly reduce engine wear including corrosive wear. This is especially true in the later half of an oil drain period when the lubricant's acid-neutralizing capability has been depleted.